The present invention relates to extruded thermoplastic foams.
Rigid foam sheets have a variety of applications, including feedstock for thermoforming applications, such as cups, bowls, non-flexible applications where stiffness is needed to span large distances, as in automobile headliner substrates or packaging decks, trays, etc. However, certain applications require higher heat resistance than are typically seen with foams of polymers such as polyethylene and polystyrene. Polyurethane foam is a foam typically employed in such applications, but may not be as readily thermoformable as a foam of a thermoplastic polymer. Foam sheet made from foamable polypropylene beads are a means to obtain a thermoformable foam having a relatively high heat resistance, but the foam may not have impact resistance or flex and tensile strength properties required for certain applications. Also, polypropylene bead foams are often made from polypropylene copolymers which are more flexible than polypropylene homopolymer foams and will have a lower heat resistance property as compared to foams made from homopolymer polypropylene.
U.S. Pat. No. 3,637,458 discloses microcellular foam sheet made by an extrusion process from polypropylene polymer, but the use of relatively large amount of blowing agent in such processes produces a foam which may not have the desired density, thickness, or stiffness needed for certain applications.